1. Field of the Invention
This invention relates to new fluorine-containing copolyimides and copolyamide-acids which exhibit improved solubility in common organic solvents, low dielectric constants and improved thermal flow properties.
2. Description of Related Art
Polyimides are widely used in the aerospace industry and electronics industry, because of their toughness, low density, thermal stability, radiation resistance and mechanical strength. However, it is recognized that polyimides are difficult to process. The processing problems arise from the insolubility of polyimides in most of the more common solvents. Consequently, products have been fabricated from polyamide-acid intermediates, which are more soluble but less stable, and then imidized by the application of heat to provide the desired end product. The disadvantage of this process is that the water liberated during the imidization of the polyamide-acid forms undesirable voids or surface irregularities in the final product which reduces its mechanical properties.
It has been suggested that polyimides having hexafluoroisopropylidene linking group in the diamine and/or dianhydride comonomers have improved solubility properties. Several patents disclose polyimides prepared from diamines of this type. For example, U.S. Pat. No. 3,356,648 to Rogers discloses polyimides prepared from 2,2-bis(4-aminophenyl) hexafluoropropane; U.S. Pat. No. 4,592,925 to DuPont et al. discloses polyimides prepared from 2,2-bis(3-aminophenyl) hexafluoropropane; U.S. Pat. No. 4,111,906 to Jones et al. discloses polyimides prepared from 2,2-bis[4-4-aminophenoxy)phenyl]hexafluoropropane; and U.S. Pat. No. 4,477,648 to Jones et al. discloses polyimides prepared from 2,2-bis[(2-halo-4-aminophenoxy)phenyl]hexafluoropropane. In addition, U.S. Pat. No. 4,592,925 discloses polyimides prepared by reacting 2,2-bis(3-aminophenyl) hexafluoropropane and 4,4'-hexafluoroispropylidenebis (phthalic anhydride), also known as 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride.
It is also known in the art to prepare polyamides by condensing a mixture of diamines, including a 3,4'-diamino diphenyl compound, with a dicarboxylic acid, such as disclosed in U.S. Pat. No. 4,075,172. Similar polyamides are suggested in U.S. Pat. No. 3,792,148. U.S. Pat. No. 3,328,352 teaches polyamides containing hexafluroisopropylidene linking groups.